Data coupling apparatus is interposed in a transmission medium at a data station to enable some function to be performed by or on the data. The data from the transmission medium is routed through the data station and data manipulation means for performance of the designated function and then back onto the transmission medium.
For high density usage applications, it is common for the transmission medium to carry a plurality of multiplexed digital channels thereon in a framed format. For example, a multiframe may contain a plurality of frames, and each frame may contain a plurality of channels. The coupling apparatus includes an input which delivers the received data to data manipulation means for performance of the desired function. The data is then delivered to a data buffer storage element providing a delay. The coupling apparatus includes an output which arbitrarily selects where a word is to begin in the bit stream output therefrom to the transmission medium. This output driver sends these arbitrarily chosen word identification signals to the data buffer. The delay in the data buffer accommodates the difference in the bit stream location of word identification signals from the input and word identification signals from the output, i.e. the data input to the buffer is stored or delayed until it falls into alignment with the word identification bit location assigned by the output driver. The data buffer is thus needed because the word identification bit location assignment used by the output driver is not the same as the word identification bit location assignment used by the input as extracted from the incoming data.
Besides the need for a data buffer, the above described coupling apparatus is not suitable for substitutional insertion of data into the bit stream through the manipulation means. This is because the clock driving the manipulating means is not oriented with the data words input thereto, resulting in loss of frame alignment. The location of the desired insert channel in the bit stream is thus not known, and this ambiguity is further compounded by the delay in the data buffer noted above.